1. Field of the Invention
This invention relates to a low-cost method of producing a fine copper powder suitable for use as filler in a conductive paste or the like.
2. Background Art
Conductive pastes are widely used for forming electronic circuits and the external electrodes of ceramic capacitors. Typical conductive fillers used in conductive pastes include copper, nickel, silver and the like. Among these, copper is used extensively nowadays because it is inexpensive, low in resistance and excellent in anti-migration property. A conductive filler comprising a mixture of copper powders of various particle diameters is usually used in a conductive paste for the external electrodes of a ceramic capacitor. However, in order to form a dense film for improving electrode reliability, the copper powder prior to mixing needs to be one of high fineness, e.g., of a particle diameter of not greater than 1 μm or even not greater than 0.5 μm, and of uniform particle size.
Methods available for copper powder production include, for example, the atomization process, mechanical crushing process, electrolytic deposition process, vapor deposition process and wet reduction process. The wet reduction process is the main one used today because it is superior in the point of enabling efficient production of a copper powder that is composed of fine spherical particles having a narrow particle size distribution and, as such, is suitable for use in a conductive paste. For example, the prior art includes processes for obtaining fine copper powder by using hydrazine to reduce copper oxide, as taught by JP 10-330801A (Ref. No. 1), JP 1-290706A (Ref. No. 2) and JP 5-57324B (Ref. No. 3).
As can be seen from Ref. No. 2, reaction control is generally difficult in a method of reducing a bivalent copper oxide directly to copper metal because the (2-valent→1-valent) and (1-valent→0-valent) reactions progress in parallel. A copper powder of the desired particle diameter and particle size distribution is therefore hard to obtain. In response to this problem, Ref. Nos. 1 and 3 teach production of a spherical copper powder of narrow particle size distribution by reducing and precipitating homogeneous monovalent copper oxide (cuprous oxide) from bivalent copper oxide and then producing the final copper particles by a further reduction reaction. However, this prior art method is a two-stage reaction process including a first-stage reduction reaction for precipitating cuprous oxide and a second-stage reduction reaction for precipitating copper metal from the cuprous oxide and further requires liquor removal, water washing and other process to be carried out in the course between these stages. It thus consists of numerous steps and requires a long time to complete. In addition, the production cost is high owing to the need to use more than one reducing agent.
On the other hand, cuprous oxide, which is an intermediate product of the prior art production method, is produced on an industrial scale as a relatively inexpensive and high grade product among copper compounds. If, instead of the foregoing method, there should be practically applied a copper powder production method making direct use of such cuprous oxide as the starting material, it would be possible to complete the reduction in a single stage and as a result to realize improved productivity and lower cost.
However, cuprous oxide generally available for industrial purposes is manufactured by the electrolytic method. Cuprous oxide produced by this method has an average particle diameter of several μm, is of irregular particle shape, and varies in particle size distribution.
The diameter of copper particles obtained by reducing cuprous oxide ordinarily depends on the particle size distribution of the cuprous oxide. When cuprous oxide of large particle diameter is used, the particle diameter of the copper particles is large, and when cuprous oxide of small particle diameter is used, the particle diameter of the copper particles is small. Copper powder of uniform particle diameter is therefore difficult to produce with good reproducibility when electrolytic cuprous oxide is used as the starting material without further processing.
It is true that a fine copper powder can be obtained with electrolytic cuprous oxide as the starting material by adopting a measure such as adding a large amount of surfactant or refining the electrolytic cuprous oxide to a particle diameter of, say, 0.5 μm or smaller by subjecting it to crushing treatment beforehand. However, such measures cannot be easily adopted because they lead to increased cost.